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An element in the alkali metal group. Cesium has an abundance of 1 ppm in the earth's crust. It occurs in the minerals pollucite and lepidolite. Cesium was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff. It is a soft, silvery white metal that tarnishes on contact with air and can ignite spontaneously in moist air. Cesium is used as a catalyst for polymers and as a radioactive emitter in atomic clocks. It is also used in photoelectric sensors for security devices and cameras.

Synonyms and Related Terms

Cs; caesium (IUPAC); cesium (US,. Ned., Sven.); césium (Fr.); Cäsium (Deut.); cesio (It., Esp.); Césio (Port.)


  • Metallic cesium can ignite spontaneously in moist air.
  • Dangerous fire and explosion risk.
  • Reacts violently with oxidizing materials.
  • Burns skin.
  • Fisher Scientific: MSDS

Physical and Chemical Properties

  • Flame color is blue to purple.
  • Violently decomposes in water with the evolution of hydrogen.
  • Soluble in liquid ammonia.
Composition Cs (atomic no. 55)
CAS 7440-46-2
Mohs Hardness 0.2
Melting Point 28.5 C
Density 1.873 g/ml
Molecular Weight atomic wt. =132.9054
Boiling Point 705 C

Resources and Citations

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 180
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 2051
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998

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